1. Field of the Invention
The invention relates to a stable composition comprising zeolite nanocrystals and zeolite precursor nanoparticles for producing an abrasion-resistant anti-reflection layer, having a refractive index close to the optimum refractive index of 1.22, on substrates, preferably glass, and to a process for the preparation of such composition. The anti-reflection layer reduces the reflection of light over a broad spectrum, which is particularly suitable for protective cover of flat panel display or photovoltaic solar panels.
2. Description of Related Art
Anti-reflective (AR) film is generally disposed on an outermost surface of an image display device, such as the polarizing film for a liquid crystal display (LCD), the front plate of a touch panel (PET substrate), the front plate of a projection television (PC substrate), the front plate of a cathode ray tube display or plasma display panel (glass substrate), to reduce reflectance and prevent optical interference or image glare caused by external light or enhancing the visibility of image. It is also needed in the solar panel covering glass to enhance the penetration of incident light.
There are several approaches to produce the desired anti-reflection effect. The first approach is to coat a multiple-layered stack with alternating high and low refractive index materials. To achieve the desired anti-reflection effect, a tight thickness control of each layer is needed, so that the destructive interference occurred at the target wavelength range. However, the processing of multilayer is cumbersome and thus the productivity is low. There are increasing needs to find an easier alternative.
Another alternative may be to create a gradient refractive index along the thickness of the coated film. Such gradient index is known to produce a broadband anti-reflection effect. Particularly, a single-layer antireflective film having a gradient refractive index can be obtained by various methods, such as etching, sol-gel, phase separation, micro-imprinting, or molding. In the extreme case, a single layer of ˜110 nm silica particle electrostatically anchored to the substrate via polyelectrolyte had been reported to reduce >90% of reflection in visible range (H. Hattori, Adv. Mater. 2001, 13, 51-54).
Besides the conventional multiple coatings and gradient refractive index layer, it is also possible to generate an anti-reflection action by means of a single coating. The simplest design of an single layer AR film would be just a monolayer with a refractive index (n) of n=(no*nsub)1/2, where no is the refractive index of air and nsub is the refractive index of the substrate, and with an optical thickness of λ/4, where λ is the wavelength where the reflection is to be minimized. For glass substrate with a refractive index of 1.52, this means a refractive index of 1.23, and a thickness of 110 nm to reach zero reflection at ˜540 nm. The most-used anti-reflection monolayer of this type is a λ/4 layer of MgF2 having a refractive index of 1.38 applied by vapor deposition.
To achieve a refractive index below 1.3, the only possibility is to using porous materials. Typically, a porous AR coating is made of silica sol-gel with sacrificed porogen to create nano-pores, such as that disclosed in U.S. Pat. Nos. 6,918,957 and 7,128,944. In these inventions, a hybrid sol comprising surfactants and 10-60 nm sized silicon oxide hydroxide nanoparticles was coated on glass. Subsequent removal of the organics at 600° C. produces a porous layer capable of anti-reflection effect. To improve the abrasion resistance, U.S. Pat. No. 7,241,505 further described the partitioning of these silicon oxide hydroxide nanoparticles into two size fractions with specific weight ratios. In the above-mentioned patents, the silicon oxide hydroxide nanoparticles were made from a process where tetraalkoxysilane was added to an aqueous-alcoholic ammoniacal hydrolysis mixture.
For porous silica film to achieve the desired refractive index, roughly 58% porosity is needed. The mechanical strength would be impaired if the skeleton is not strong enough. Zeolite, a crystalline tectosilicates having a low refractive index of ˜1.3 (S. Nair, M. Tsapatsis, Micropor. Mesopor. Mater. 2003, 58 81-89) and an elastic modulus above 30 GPa (Z. J. Li, M. C. Johnson, M. W. Sun, E. T. Ryan, D. J. Earl, W. Maichen, J. I. Martin, S. Li, C. M. Lew, J. Wang, M. W. Deem, M. E. Davis, Y. S. Yan, Angew. Chem. Int. Edn 2006, 45 6329-6332), would be an ideal porous silica to be used for AR coating.
U.S. Pat. No. 7,381,461 described an antireflective transparent zeolite hardcoat, comprising a zeolite nanostructure made of zeolite nanocrystals vertically stacked into a porous structure on a substrate, wherein the porosity increases with structure height, thereby providing a smooth refractive index transition.
In order to obtain a transparent layer of zeolite, U.S. Pat. No. 7,253,130 described a method of preparing a precursor sol capable of forming zeolite, coating the precursor sol to a surface of a substrate, and heating the coated substrate under a temperature between about 120° C. and about 250° C. under a humidity less than a saturation to convert the precursor sol to a transparent zeolite film. However, the conversion of zeolite precursors into zeolite via heating in humidity is rather time consuming.